题干
— Do you have email address?
— Sorry, I only have a Wechat(微信) ID.
A：a
B：an
C：the
D：/
答案(点此获取答案解析）
B
— Do you have email address?
— Sorry, I only have a Wechat(微信) ID.
A：a
B：an
C：the
D：/
满足：
如果
为数列
的前n项和，那么
的概率为 